The present invention is related to a technique for reducing the digital transmission rate of speech signals while at the same time maintaining a high quality signal in a digital communications system. More particularly, the present invention is related to a "nearly instantaneous companding" (NIC) source coding technique which uses a rescaling protocol on a block of samples to reduce the transmission bit rate. Examples of such coding techniques may be found in "Near Instantaneous Companding for Non-Uniformly Quantized PCM", by D. L. Duttweiler and D. G. Messerschmitt in IEEE Transactions on Communications, Vol. COM-24, No. 8, August 1976, at pages 843-864; and in "Nearly Instantaneous Companding and Time Diversity as Applied to Mobile Radio Transmission", in International Conference on Communications, June 1975, San Francisco, at pages 40(12)-40(15).
With reference to a prior art compander shown in FIG. 1, digital amplitude samples from a speech signal are applied to an A-law or .mu.-law compander 10, where the digital amplitude samples are compressed according to the well known A-law or .mu.-law data compression schemes. The digitally companded samples are applied on the one hand to an N Sample Memory 15, while on the other hand applied to Maximum Sample Finder 20. The Maximum Sample Finder 20 determines the maximum value of the N digitally companded speech samples stored in memory 15, and delivers the maximum value on the one hand to quantizer 25 and on the other hand to Encoder 30.
Generally, the purpose of the NIC processor is to reduce the transmission bit rate of the digitally companded signal x(nT), where n is equal to 0, 1, 2 . . . , and T is equal to 125 .mu.sec, for example. For convenience, the signal x(nT) will hereinafter be denoted x.sub.n. The reduction in transmission bit rate is accomplished by collecting the block of N samples in memory 15, determining the largest sample magnitude, S.sub.max, among the N samples to thereby select the quantizing range and uniformly quantizing the N samples in quantizer 25.
The quantizing step size .DELTA..sub.i used in the quantizer 25 is given by EQU .DELTA..sub.i =S.sub.max /2.sup.p-1, (1)
where p represents the number of bits/sample used in the transmission of the sampled information. The samples quantized in accordance with the quantizing step .DELTA..sub.i are then encoded in encoder 30.
In addition to the encoded digital speech samples, m bits are needed to transmit the protocol information, namely the value S.sub.max or .DELTA..sub.i, to the receiver in order to decode that particular block of N samples. Thus, if S.sub.max is transmitted, the protocol information bit rate for A-law companded signals having seven bits of precision (plus one sign bit) would be 7 S/N bits per second, where S is the sampling rate. The total transmission rate is defined as the sum of the nominal rate and the protocol rate.
Thus, the overall transmission bit rate can be reduced without an overall degradation in speech quality since the quantizing or signal measuring process automatically accommodates wide variations in the value of signal amplitudes. However, this method does not take advantage of the probability of occurrences in speech signal amplitudes, since it uses the same number of quantizer levels (2.sup.p-1), wherein the number of quantizer levels per sample block will hereinafter be referred to as the .-+.byte length" regardless of the value of S.sub.max. Furthermore, the higher the value of S.sub.max, the larger the value of .DELTA..sub.i. Thus, the value of .DELTA..sub.i affects the quantizing noise which increases with an increasing .DELTA..sub.i. Again, the value of p in equation (1) cannot be varied in accordance with the above mentioned technique.